Series arc faults occur when loose or contaminated contacts begin arcing. The arc current is in series with the load and therefore limited to the load current. Common causes of series faults are contact overloading, the installation of a wrong pin type in a pin and socket connector, vibration loosening of contacts, and chemical or physical changes to the contact surfaces. Evidence that series arcing has occurred includes pitted contacts, heat damage to the insulation around the contacts, and overheating of the wires leading to the fault.
Often the only indication of an intermittent series fault is erratic operation of the load and/or radio interference caused by the arcing. When a pilot reports such symptoms and a ground technician attempts to reproduce them, it is not uncommon that everything will appear to be working fine. Often only particular environmental conditions (e.g., temperature, run time, load current, vibration, etc.) will get it arcing and cause it to fail in a noticeable manner. A contact that has been arcing, however, will generally exhibit higher than normal contact resistance when current is flowing through it. This is because arcing leaves tiny metallic bridges that constrict current flow and therefore show increased resistance. These bridges are chaotically broken and reformed from the heat of the arc during periods when it is arcing. In a subsequent stable period, the current is carried through one or two of these constricted bridges which, because they constrict the current flow, heat up. As their temperature rises, the resistance and therefore voltage across them increases. It is thus possible to discover a non-arcing series fault by measuring the voltage drop across each set of contacts under load.
A technician might diagnose such a problem by first measuring the voltage drop over the entire length of the cable which encompasses all the connections in between. The normal “acceptable” voltage drop under load might be on the order of a few millivolts. Much higher readings are indicative of a bad connection. If higher readings are found, the next step would be to locate the faulty connection by measuring the voltage drop across each connector progressively down the line. Or alternatively one might use a binary search, i.e., measure the drop from the halfway point to isolate it to one half or the other, then to successive halfway points until the fault is located. In either case, the principal problem the technician faces is that he cannot access the conductors to make the measurements. If he unplugs the connector to access the pins, he disconnects the load. Plugged in, the wires and contacts are typically not accessible. Even in the case where one can remove the connector shell to access contacts, doing so with power applied is not safe or practical. There exists a need, therefore, for a diagnostic means and method whereby such voltage drop measurements could be made is in situ under load and without access to the conductors.